According to mathematical knowledge, we know that a straight line divides a plane into two parts at most, and two straight lines divide a plane into four parts at most. How about three? How many parts can n straight lines divide a plane at most? Let n straight lines divide a plane into s parts at most, and get the following formula by reasoning: S=(n*n+n+2)/2. When the s value is greater than or equal to 56, it is easy to get that the n value is 10 and the s value is exactly 56, so at least 10 knife is cut on the cake (when there is a focus between the two knives).